Hair coloring agents presently available fall into one of several basic categories, depending on how they interact with the hair shaft and how long they last. Temporary hair colors coat the surface of the hair shafts and typically wash out within several shampoos. Semipermanent dyes penetrate into the hair shaft, but not as deeply as permanent dyes. While the semipermanent dyes do not rinse off with water, they fade over time and they can be washed out of the hair after about five to ten shampoos. Gradual or progressive dyes (e.g., Grecian Formula 16), usually marketed to men, contain lead acetate [Pb(CH3COO)2], which penetrates the hair cuticle. The Pb2+ ions react with sulfur atoms in the hair shaft proteins to form lead sulfide (PbS), which is a dark color. More frequent use leads to an increased darkening of the hair. Finally, permanent hair dyes are available for long-lasting color change. These formulations penetrate the hair shaft to deposit pigments therein, so they cannot be washed out. Before the coloring agents of a permanent hair dye can penetrate the hair shaft, the cuticle of the hair shaft must be opened so that the pigments can gain access to the interior.
For example, a permanent hair-color product may consist of two chemical components that are packaged separately and mixed together immediately before being applied to the hair. One component can be a solution of hydrogen peroxide in a water or emollient base. The second component can be an ammonia-based solution of dye intermediates and/or preformed dyes or couplers. Formulations for permanent dyes generally include oxidative hair dye precursors that diffuse through the cuticle of the hair and into the cortex, where they can react with each other in the presence of oxidizing agents, thereby forming larger dye molecules. The large size of the dye molecules prevents them from diffusing out of the cortex, so that the hair color change is permanent. These reactions typically require a pH of approximately 10 in the presence of an oxidizing agent.
Exemplary dye intermediates include small molecules such as ortho- or para-diaminobenzenes, aminohydroxybenzenes, and to a lesser extent dihydroxybenzenes that develop color on oxidation. The color couplers include small molecules such as various phenols, meta di-substituted phenylenediamines and phenyleneaminophenols, and certain various resorcinol (1,3-dihydroxybenzene) derivatives. The couplers do not oxidize readily, but they react with the oxidized primary intermediates to yield a greater range of colors.
Under typical circumstances, the hydrogen peroxide solution and the ammonia-dye solution are mixed together and promptly applied to the hair. The ammonia in the mixture causes the hair shaft to swell, thereby separating the scales of the hair cuticle. When this happens, the dye precursors can penetrate the cuticle. The dye precursors complete their chemical reactions once they have gained access to the interior of the hair shaft. Dyes bind covalently to proteins within the hair shaft.
Variations in hair color may be obtained by varying the dye intermediates. For instance, higher concentrations of certain intermediates may yield darker hair colors. Adding resorcinol as an intermediate may make a color more yellow, while adding 4-amino-2-hydroxytoluene will make the color redder. Dyes added along with the oxidation dye intermediates may make a tone brighter or more intense. For example, using a dye like 2-nitro-p-phenylenediamine may yield a bright orange color.
Many of the chemicals used in permanent dye systems have potential toxicities. As an example, the primary amine used in oxidative dye systems is generally paraphenilene-diamine (PPD). There are concerns that PPD may be implicated in carcinogenesis and mutagenesis. As another example, there is evidence of toxicity from dye system precursors such as the coupler resorcinol. Furthermore, the high pH and the use of hydrogen peroxide lead to conditions that damage the hair shaft and irritate the scalp and skin.
Certain chemicals used in semi-permanent hair color systems have potential toxicities as well. As an example, coal tar dyes are used for semi-permanent hair coloring. Many coal tar dyes and mono-azo/di-azo dyes that yield red and red-orange shades are suspected of carcinogenesis, teratogenesis and/or mutagenesis. Dyes used in semi-permanent preparations may also stain scalp and skin.
Natural dyes have disadvantages for hair coloring when compared to the synthetic preparations described above. A major disadvantage of natural dyes is their tendency to produce dull color shades. In addition, natural dyes are shampoo-resistant than synthetic preparations. Plants provide natural sources for dyes, but contain low concentrations of appropriate colorants. Moreover, dyes that are retrieved from plants may be impure and/or may have poor solubility. As a result, natural dyes may require prolonged application times to yield desirable, natural-looking hair color. In addition, most natural hair dyes are provided as powders, which are inconvenient for consumers or hairdressers to apply.
There remains a need in the art, therefore, for safe and effective hair coloring products and processes. There remains a further need in the art for hair color formulations that use safe, non-toxic and environmentally benign materials.